In the anime

History

Kanto

Muk in its debut

In Sparks Fly for Magnemite, Ash and his friends were traveling through Gringey City, which was in the midst of a power failure. The local power plant was being overrun by the mysterious Poison-typePokémonGrimer. After the swarm was located, the Grimer's leader, a Muk, came and challenged the group. However, the vast numbers of Magnemite and Magneton in the city combined attacks to clear out the Grimer, leaving only Muk. With the aid of Pikachu and a Magnemite that was following it due to an electrical build up, Ash defeated and captured the Muk. It was then immediately sent to Professor Oak's Laboratory because its stench was so rancid that it could seep through the Poké Ball. However, Muk's horrid smell seemed to have disappeared by its later appearances as neither Ash, nor anyone around him, was affected by it.

Muk was reunited with Ash when he returned home in A Tent Situation. Muk insisted on fighting Team Rocket, and Ash ordered it into battle instead of choosing Bulbasaur. Muk managed to resist Razor Leaf attacks and a Poison Powder from James's Victreebel. Lickitung's Lick was even less effective, wiping Muk's face ineffectively to the point of causing pleasure instead of damage. It was then put to sleep by Sleep Powder, but awoke after Gary intervened and drove Team Rocket away.

Personality and characteristics

Muk showing affection for its Trainer

Ash's Muk is an extremely happy and affectionate Pokémon. It is particularly affectionate towards Professor Oak and as a recurring gag first seen in Who Gets To Keep Togepi?, it would always smother the Professor whenever Ash would make a call to the lab.

Muk loves humans and it's not too particular what kind.

Muk's affection also extends to its Trainer, second in enthusiasm only to Bayleef, despite only having fought for him on three occasions. Muk obligingly crushes Ash in excitement whenever Ash returns to the lab or summons it to his party. Its affection also extends to complete strangers, as was seen when it completely ignored Jessie's Lickitung in order to hug Jessie, also smothering James's Victreebel in excitement. It also smothered Professor Birch upon meeting him in The Right Place and the Right Mime. Professor Oak suggests that Muk's overwhelming displays of affection is simply because it loves humans.

Muk differs from most of Ash's other Pokémon in that it is a defensive Pokémon, with a low attack power and a small moveset. It has never been shown to be damaged by a physical attack, with its opponents' moves always absorbed by its sludge body. Despite being quite powerful and good in battle, it has only ever defeated one opponent: Jeanette Fisher's Bellsprout.

In the manga

In the Ash & Pikachu manga

In The Electric Tale of Pikachu manga

Ash caught Muk off-screen and later used it as his last Pokémon is his battle against Jeanette in Welcome to the Big Leagues. Jeanette's Bellsprout had no chances of using its techniques against Muk's slimy body, and Ash won the battle.